In power plants, generators are coupled for example to a transmission bus or a grid through step-up transformers equipped with On-Load Tap Changers (OLTC), sometimes including an automatic tap control function.
This allows changing taps and thus the transformation ratio of the step-up transformer. It is common practice to use on-load tap changers to adjust the reactive power output of a generating unit when it is synchronized to the grid. The output of reactive power is often requested by the transport grid operator, and may be specified e.g. in a grid code, or on day-to-day basis.
On the other hand, it is desirable to keep the voltage on the generator terminals constant at its nominal value. This has the advantage that the generator has its full dynamic regulating capability, and the station supply, that is often connected to the generator terminals, is operating at nominal voltage as well.
These criteria can be met, if an automatic voltage regulator (AVR) in the excitation system of the generator maintains the generator voltage at nominal value, while the reactive power supplied to the grid is adjusted via the OLTC.
On-load tap changers are severe duty motor-driven switching systems, requiring arcing contacts and bypass contacts to ensure continuity while switching from one tap to another. The voltage change from one tap to another is typically less than 1%, and OLTCs have quite several tens of taps to provide a reasonable range of operation.
The control of OLTCs of step-up transformers is today commonly done automatically upon voltage measurements on the output side of the step up transformer, employing an automatic voltage regulator associated to the OLTC. The reference value for the voltage regulator has to be set manually by the operator staff, until the specified reactive power output to the grid is achieved. As soon as the grid condition or the operating regime of the generator changes, the reference value has to be re-adjusted.
In the state of the art, the step up transformer and its OLTC is equipped with a proper regulator. This regulator is an automatic voltage regulator (AVR) coupled with an input to the transformer output voltage and driving the motors of the OLTC automatically.
This known solution, which requires dedicated hardware for the OLTC regulator and instrument transformers for measuring the secondary current and voltage of the transformer, is quite cost intensive.
In addition, dysfunctions in conjunction with long term voltage drops in the transmission grid have been observed.
The present applicant has studied in detail these phenomena and could establish surprisingly, that they are due to uncoordinated interactions of generator excitation limiters and OLTC control.
When the generator over-excitation limiter (OEL) reduces the excitation current in the generator to prevent excessive thermal load in the rotor and/or stator windings, the terminal voltage of the generator is reduced. As a result, the station supply of the unit may become unstable and eventually trip the whole unit.
Furthermore, the OLTC control may compromise stable operation of the generator in the under-excited operating regime, if not co-ordinated with the generator excitation control.